Field bus arrangement with a field bus distributor

ABSTRACT

The invention relates to a field bus arrangement with a field bus distributor ( 11 ) for mounting in the area ( 1 ) at risk of explosion, for feeding a plurality of field devices ( 121  to  12 n), arranged in the area ( 1 ) at risk of explosion, via intrinsically safe electric circuits ( 13 ) from at least one high-energy voltage source ( 21 ) arranged in the area ( 2 ) not at risk of explosion. For this purpose, in the area ( 2 ) not at risk of explosion, a field bus coupler ( 22 ) arranged in the area ( 2 ) not at risk of an explosion and having three electric circuits which are DC-isolated from one another is provided.

DESCRIPTION

The invention relates to a field bus arrangement with a distributor forfield bus systems, which distributor can preferably be mounted in thearea of a process engineering installation that is at risk of explosion.

Field buses are electrical apparatuses for communication between, on theone hand, field devices arranged in the area at risk of explosion, theso-called field area, examples of said field devices being sensors,actuators and measurement transducers, and, on the other hand, open-loopand closed-loop control systems arranged in the area not at risk ofexplosion, the so-called control room area. A field bus is usuallydesigned as a two-wire line which simultaneously serves for transmittingthe supply power for the connected field devices. In this case, thecommunication is effected in an analog fashion via a 0/4 . . . 20 mAcurrent loop or digitally, for example by means of FSK modulation, or bycombined analog/digital signal transmission.

For use in areas that are at risk of explosion, particular requirementsare made of electrical operating equipment in order to preclude apossible explosion accident. In the context of installing andcommissioning electrotechnical apparatuses and equipment and in thecourse of maintenance work on electrotechnical apparatuses and equipmentin process engineering installations which, because of their intendeduse, are located in an atmosphere at risk of explosion, the observanceof relevant statutory provisions such as the “Regulation on electricalinstallations in areas at risk of explosion—ElexV”, which is based onthe European standards for explosion protection EN 50 014 ff., isbinding.

On account of these statutory provisions, intrinsically safe electriccircuits, exclusively, said circuits being subject to the rules of EN 50020 as “intrinsic safety” type of protection, are allowed tounconditionally disconnect and connect electrical lines in the course ofoperation.

Intrinsically safe electric circuits are subject to power limitation,however, and are thus unsuitable for the connection of devices andapparatuses with a power demand which exceeds the predetermined limitvalue.

In the case of all other types of protection, prior to manipulation onelectric circuits, the entire installation must be disconnected from thevoltage supply to an extent such that metallic components of therelevant electric circuits are isolated from voltage. In this case, theprogress of the process proceeding on the process engineeringinstallation is interrupted for the period of time between disconnectionfrom the voltage supply and restart. The process then has to be startedup anew. Operators of such process engineering installations find theseinterruptions extremely disruptive.

Safety barriers of multichannel design are known, e.g. from MTL (4700series), in which each measurement channel is equipped with separatecurrent limiting and separate voltage limiting. Safety barriers areregularly arranged in the area not at risk of explosion.

Furthermore, the patent specification DE 44 03 961 C2 describes a supplysystem for an intrinsically safe field bus, in which the means forcurrent and voltage limiting are arranged such that they are spatiallyseparate. What is disadvantageous in this case is the certification inaccordance with the applicable statutory provisions on account of theinclusion of a plurality of devices in the certificate of conformity forexplosion protection.

The invention is based on the object, therefore, of specifying a fieldbus arrangement with a field bus distributor—which can be mounted in thearea of a process engineering installation that is at risk ofexplosion—for providing a multiplicity of connecting terminals ofintrinsically safe electric circuits for power supply and datatransmission for field devices having a bus capability, in which casethe field bus distributor can be expanded to form an operationallyungrounded safety barrier.

According to the invention, this object is achieved by the means ofPatent Claim 1. Advantageous refinements of the invention are specifiedin the claims referring back thereto.

The basic concept of this invention is the conversion and distributionof a large electrical power for supplying energy to field devices andalso the reduction and limiting thereof to currents and voltages whichenables the connection of certified explosion-protected operatingequipment.

To that end, a field bus distributor which realizes the reduction ofcurrent and voltage to intrinsically safe values is arranged in the areaof a process engineering installation that is at risk of explosion, thenumber of field devices that can be connected being as high as possible.

In detail, a high-energy voltage source is arranged in the area not atrisk of explosion, the output power of which voltage source can bedimensioned independently of limitations prescribed by types ofprotection in accordance with the number of field devices to beconnected and the line requirement thereof.

Furthermore, the main field bus run carrying exclusively communicationsignals appertaining to the data transmission is laid in the area not atrisk of explosion.

The main field bus run and the voltage source are connected to a fieldbus coupler, which is preferably arranged in the area not at risk ofexplosion. The field bus coupler has three electric, circuits which areDC-isolated from one another, of which a first electric circuit isconnected to the main field bus run. The field bus coupler is connectedto the voltage source via a second electric circuit. The third electriccircuit is routed as a non-intrinsically safe electric circuit into thearea at risk of explosion and the supply voltage of the voltage sourceand the communication signals appertaining to the data transmission areapplied to said third electric circuit.

The conductors of the non-intrinsically safe electric circuit areconnected to terminals of a protected connection for connection of anon-intrinsically safe electric circuit of a field bus distributorarranged in the area at risk of explosion. This field bus distributor isequipped with n connections for intrinsically safe electric circuits forconnection of a plurality of field devices. Each of the n connectionsfor intrinsically safe electric circuits is connected to a busbararrangement via a current limiting means. The protected connections forconnection of a non-intrinsically safe electric circuit are connected tothe busbar arrangement via voltage limiting means.

The field bus distributor has a housing-like encapsulation enclosing allthe live components as a whole.

Each intrinsically safe electric circuit proceeding from the field busdistributor carries a voltage- and current-intensity-limited supplypower for the connected field devices and superposed communicationsignals appertaining to the data transmission.

It is advantageous that the outlay on cabling between apparatuses in thearea not at risk of explosion and apparatuses in the area at risk ofexplosion is very low. In this case, the subject matter of the presentinvention avoids both the voltage limiting for each measurement channelin the case of multichannel safety barriers and the inclusion of aplurality of devices in a certificate of conformity, since, in thecontext of the field bus arrangement which is the subject matter of theinvention, only the field bus distributor is inherently the subject ofofficial certification. Consequently, the technical outlay for attainingthe certification that is prescribed for the use of electrical operatingequipment in the area at risk of explosion is limited to the minimumthat is technically necessary.

The voltage limiting means are constructed from zener diodes which areredundantly reverse-connected in series and are connected in parallelwith the non-intrinsically safe electric circuit and upstream of whichat least one fuse is connected on the supply side. By virtue of thegrounding of the node of the interconnection of zener diodes, thevoltage limiting means can be expanded to form an operationallyungrounded safety barrier in a simple and advantageous manner and withadditional components being dispensed with.

The invention is explained in more detail below using exemplaryembodiments. In the drawings required for this:

FIG. 1 shows a basic illustration of a field bus arrangement

FIG. 2 shows a detailed illustration of a field bus distributor

FIG. 3 shows a detailed illustration of a field bus distributor which isexpanded to form an operationally ungrounded safety barrier.

The functional principle is evident from FIG. 1. The field bus isconveyed via a field bus coupler 22 proceeding from a main field bus run23, which is routed in the area 2 not at risk of explosion, to the fieldbus distributor 11 in the area 1 at risk of explosion. In this case, thefield bus coupler 22 is arranged in the area 2 not at risk of explosionand comprises a standardized physical interface for coupling the fieldbus which is routed into the area 1 at risk of explosion. The requiredpower supply via the field bus is also provided via the field buscoupler 22.

The main field bus run 23 is connected to a process control system or astored-program controller in which measurement data received via thefield bus are evaluated and processed and proceeding from whichmanipulated variables and configuration settings can be sent via thefield bus. The requisite transmission mechanism is defined in a widevariety of national and international standards. At present thewidespread application of a physical transmission level forinstallations at risk of explosion is the interface according, to IEC1158 Part 2, in which the data are transmitted by means of Manchestercoding.

However, in principle the invention can be applied to all field bussystems which enable transmission via a core pair. This also includesthe customary HART interface for analog measurement transducers in amultidrop circuit, the simultaneous transmission of the analog measuredvalue being dispensed with in this case.

In detail, a high-energy voltage source 21 is arranged in the area 2 notat risk of explosion, the output power of which voltage source can bedimensioned independently of limitations prescribed by types ofprotection in accordance with the number of field devices 121 to 12n tobe connected and the line requirement thereof.

Furthermore, the main field bus run 23 carrying exclusivelycommunication signals appertaining to the data transmission is laid inthe area 2 not at risk of explosion.

The main field bus run 23 and the voltage source 21 are connected to afield bus coupler 22, which is arranged in the area 2 not at risk ofexplosion. The field bus coupler 22 has three electric circuits whichare DC-isolated from one another, of which a first electric circuit isconnected to the main field bus run 23. The field bus coupler 22 isconnected to the voltage source 21 via a second electric circuit. Thethird electric circuit is routed as a non-intrinsically safe electriccircuit 24 into the area 1 at risk of explosion and the supply voltageof the voltage source 21 and the communication signals appertaining tothe data transmission are applied to said third electric circuit.

In accordance with FIGS. 2 and 3, the conductors of thenon-intrinsically safe electric circuit 24 are connected to terminals1122 and 1123 of a protected connection 1121 for connection of anon-intrinsically safe electric circuit 24 of the field bus distributor11 arranged in the area 1 at risk of explosion. The terminals 1122 and1123 of the protected connection 1121 have mechanical protection againstimpermissible manipulation of the connection between the conductors ofthe non-intrinsically safe electric circuit 24 and the respectiveterminal 1122 and 1123 in the live state of the terminals 1122 and 1123.Such terminals are known per se and described in DE 195 13 645, forexample.

The field bus distributor 11 is equipped with n connections 1111 to 111nfor intrinsically safe electric circuits for connection of a pluralityof field devices 121 to 12n. Each of the n connections. 1111 to 111n forintrinsically safe electric circuits is connected to a busbararrangement 115 via a current limiting means 1131 to 113n. The terminals1122 and 1123 of the protected connection 1121 for connection of anon-intrinsically safe electric circuit 24 are connected to the busbararrangement 115 via voltage limiting means 114.

In the simplest case, the current limiting means 1131 to 113n aredesigned as series resistors channel by channel for each intrinsicallysafe electric circuit 13. Electronic current limiting may alternativelybe provided.

The field bus distributor 11 has a housing-like encapsulation enclosingall the live components as a whole. In this case, provision may be madefor realizing the encapsulation by potting with a suitable pottingcompound known per se. As an alternative, provision may be made forrealizing the encapsulation by a sand-filled housing which encloses thefield bus distributor 11.

Each intrinsically safe electric circuit 1111 to 111n proceeding fromthe field bus distributor 11 carries a voltage- andcurrent-intensity-limited supply power for the connected field devices121 to 12n and superposed communication signals appertaining to the datatransmission.

Depending on the power demand of each field device 121 to 12n, provisionmay be made, in a particular refinement of the invention, for connectinga plurality of field devices 121 to 12n to the same intrinsically safeelectric circuit 13.

The voltage limiting means 114 are constructed from zener diodes whichare redundantly reverse-connected in series and are connected inparallel with the non-intrinsically safe electric circuit 24 andupstream of which at least one fuse is connected on the supply side. Asa result, excessive voltage increases in the non-intrinsically safeelectric circuit 24 are effectively limited to permissible valuesirrespective of their plurality.

The means for voltage limiting 114 are embodied singly in each field busdistributor 11 for all the intrinsically safe electric circuits 13proceeding from the field bus distributor 11.

By virtue of the grounding of the node of the interconnection of thezener diodes in accordance with FIG. 3, the voltage limiting means 114can be expanded to form an operationally ungrounded safety barrier in asimple and advantageous manner and with additional components beingdispensed with. The effect of this is that the reliable and completeisolation of the field bus from the supply network [lacuna].

For simple voltage limiting in accordance with FIG. 2, provision mayalternatively be made for connecting two bipolar zener diodes inparallel. Furthermore, provision may be made for realizing the voltagelimiting using other suitable means such as integrated voltageregulators or thyristor circuits.

Such field bus couplers 22 are known per se and can be supplied asancillary equipment for field bus instrumentation.

To ensure that the largest possible number of field devices can beconnected to the field bus coupler, a large electrical power must berouted into the area at risk of explosion. By way of example, theprovision of a useful power of approximately 320 mA*16 V=5.12 W isdesirable if, given connection of n=32 field devices, a current ofapproximately 10 mA is required for each field device. Such powers anduseful currents are incompatible with the “intrinsic safety” type ofprotection and are therefore routed with increased safety into the areaat risk of explosion.

In practice, a core pair is laid as bus line to the field busdistributor 11 mounted in the area 1 of a process engineeringinstallation that is at risk of explosion. The bus line transmits, via anon-intrinsically safe electric circuit 24, the necessary energy forsupplying the measurement transducers, sensors and actuators 121 to 12nand it transmits the digitally coded measurement data from the processproceeding in the area 1 at risk of explosion into the control roomarranged in the area 2 not at risk of explosion, where a stored-programcontroller or a process control system performs the evaluation andprocessing of the measurement data. Different operating modes of themeasurement transducers, sensors and actuators 121 to 12n can also beset via the field bus.

The safety-relevant voltage, current and power routed on the field busis initially not upwardly limited. Therefore, this two-wire line isdesigned with the aim of explosion protection in the “increased safety”type of protection. This likewise applies to the connection and theencapsulation of the field bus in the area 1 at risk of explosion withinthe field bus distributor or the distributor housing. The “intrinsicsafety” is ensured only after complete current and voltage limitingaccording to the provisions. Only if intrinsic safety is provided is itpossible for the user to exchange and service field devices withoutdanger in the course of the operation of an installation.

The field bus distributor according to FIG. 2 can be certifiedautonomously. All that is necessary in this case is a demonstration tothe effect that the field bus potential is reliably isolated from thesupply network by the power supply unit connected upstream. All thecomponents in the distributor housing are dimensioned in such a way asto meet the requirements of the standards.

The field bus distributor in accordance with FIG. 3 represents a safetybarrier for field bus systems which are permitted to be mounted in thearea 1 at risk of explosion and which has common voltage limiting forall the measurement channels. The requisite connection to theequipotential bonding (“ground” symbol) is present. This obviates therequirement for the field bus potential to be reliably isolated from thesupply network by the power supply unit connected upstream.

In this case, the requirements made of DC isolation of the field bus arenot determined by the explosion protection but are restricted to valueswhich are necessary only for the function.

List of reference symbols 1 Area at risk of explosion 11 Field busdistributor 1111 to 111n Connection for an intrinsically safe electriccircuit 1121 Protected connection 1122, 1123 Terminals 1131 to 113nCurrent limiting means 114 Voltage limiting means 115 Busbar 121 to 12nField devices 13 Intrinsically safe electric circuit 2 Area not at riskof explosion 21 Voltage source 22 Field bus coupler 23 Main field busrun 24 Non-intrinsically safe electric circuit

What is claimed is:
 1. A field bus arrangement comprising: a) a fieldbus distributor for mounting in an area at risk of explosion, forfeeding a plurality of field devices, arranged in said area at risk ofexplosion, by way of intrinsically safe electric circuits from at leastone high-energy voltage source arranged in an area not at risk ofexplosion, and for data transmission; and b) a field bus couplerarranged in said area not at risk of explosion, said coupler havingfirst, second and third electric circuits which are DC-isolated from oneanother, said field bus coupler connected through said first electriccircuit to a main field bus run routed in said area not at risk ofexplosion, said voltage source connected through said second electriccircuit to said field bus coupler, and said third electric circuit isrouted as a non-intrinsically safe electric circuit into said area atrisk of explosion; said field bus distributor comprising: componentscapable of carrying an electrical current, said components enclosed as awhole in a housing like encapsulation, n connections for saidintrinsically safe electric circuits for connection of said plurality offield devices, at least one protected connection for connection of saidnon-intrinsically safe electric circuit, current limiting means, voltagelimiting means and a busbar arrangement, each of said n connections forsaid intrinsically safe electric circuits is connected to said busbararrangement by way of a respective one of said current limiting means,and said at least one protected connection for connection of anon-intrinsically safe electric circuit is connected to said busbararrangement by way of said voltage limiting means.
 2. The field busarrangement of claim 1, wherein said protected connection of said fieldbus distributor for connection of said non-intrinsically safe electriccircuit has terminals with mechanical protection against impermissiblemanipulation of the connection between the conductors of saidnon-intrinsically safe electric circuit and the respective one of saidterminals when said terminals are carrying an electrical current.
 3. Thefield bus arrangement of claim 1 wherein said voltage limiting means areformed by reverse-connected series zener diodes.
 4. The field busarrangement of claim 2 wherein said voltage limiting means are formed byreverse-connected series zener diodes.
 5. The field bus arrangement ofclaim 3 wherein those connections of said zener diodes of said voltagelimiting means which are remote from said non-intrinsically safeelectric circuit are interconnected to form a node, and said field busdistributor has a grounding connection which is connected to said nodeof the interconnection of said zener diodes of said voltage limitingmeans.
 6. The field bus arrangement of claim 3 wherein those connectionsof said zener diodes of said voltage limiting means which are remotefrom said non-intrinsically safe electric circuit are interconnected toform a node, and said field bus distributor has a grounding connectionwhich is connected to said node of the interconnection of said zenerdiodes of said voltage limiting means.
 7. The field bus arrangement ofclaim 1 wherein said encapsulated components of said field busdistributor are potted with a suitable potting compound.
 8. The fieldbus arrangement of claim 2 wherein said encapsulated components of saidfield bus distributor are potted with a suitable potting compound. 9.The field bus arrangement of claim 3 wherein said encapsulatedcomponents of said field bus distributor are potted with a suitablepotting compound.
 10. The field bus arrangement of claim 4 wherein saidencapsulated components of said field bus distributor are potted with asuitable potting compound.
 11. The field bus arrangement of claim 5wherein said encapsulated components of said field bus distributor arepotted with a suitable potting compound.
 12. The field bus arrangementof claim 6 wherein said encapsulated components of said field busdistributor are potted with a suitable potting compound.
 13. The fieldbus arrangement of claim 1 wherein said field bus distributor isencapsulatedby a sand-filled housing which encloses said field busdistributor as a whole.
 14. The field bus arrangement of claim 2 whereinsaid field bus distributor is encapsulated by a sand-filled housingwhich encloses said field bus distributor as a whole.
 15. The field busarrangement of claim 3 wherein said field bus distributor isencapsulated by a sand-filled housing which encloses said field busdistributor as a whole.
 16. The field bus arrangement of claim 4 whereinsaid field bus distributor is encapsulated by a sand-filled housingwhich encloses said field bus distributor as a whole.
 17. The field busarrangement of claim 5 wherein said field bus distributor isencapsulated by a sand-filled housing which encloses said field busdistributor as a whole.
 18. The field bus arrangement of claim 6 whereinsaid field bus distributor is encapsulated by a sand-filled housingwhich encloses said field bus distributor as a whole.